The World Health Organization (WHO) is the leader when it comes to all things related to global health trends. And in a situation like the outbreak of influenza A (H1N1), also known as the human "swine flu," we look to WHO for the protocols to follow.

News of H1N1's spread has sometimes been accompanied by a chart from WHO called the "Pandemic influenza phases." The pandemic levels are graded on a scale of 1 to 6. But what does that mean? Let's decode.

First of all, it's important to know exactly what epidemic and pandemic mean. An epidemic is when there are more cases of infectious disease than normal. A pandemic means that an epidemic has gone global. It measures the scope and spread of a disease or virus - not its severity. A pandemic can spread widely but actually create quite mild symptoms.

Influenza is constantly circulating in the animal kingdom. Birds are especially susceptible, and we know they can travel far and wide as virus vectors (spreaders of the virus). We also know that at least one furry, nocturnal creature can get the flu, too. Phase 1 simply recognizes this fact and lets us know that no animal viruses currently infect humans.

Phase 2 means that some strain of animal influenza has infected a human, making it a potential pandemic threat. When a few people - or a cluster of people - become infected with a strain of influenza, we move to phase 3. Some person-to-person transmission may occur, but it's mostly among those with very close contact, like caregivers. But in this phase, the virus has not developed to the point that is easily passing from person to person.

In phase 4, a virus is easily spread from person to person, causing local outbreaks in communities. Keep in mind, though, that phase 4 is still pre-pandemic and a pandemic is not written in stone. It is at this point that WHO collaborates with affected areas to plan prevention, treatment, or containment of the virus.

A virus begins its shift into the pandemic zone in WHO's phase 5. At this point, human-to-human spread has occurred in at least two countries in one region and a pandemic is imminent. Phase 6 is considered the true "pandemic phase," with community-level outbreaks in at least one other country in a different region than was originally noted in phase 5.

If infection levels drop below peak and the spread has slowed down, we move into the post-peak period. The virus is not eradicated, and recurrent waves of infection may occur - waves of activity can occur over many months. When an infection reaches a level that's normally seen for regular seasonal influenza, we reach a post-pandemic period.